1) Field of the Invention
This invention relates to insulated wires having excellent heat resistance and good solderability and also to insulating coating compositions. More specifically, the present invention is concerned with insulated wires having a coating of a polyimide resin or an imide-modified polyurethane resin formed on conductors, production processes of the insulated wires, insulating coating compositions and flyback transformers using the insulated wires.
2) Description of the Related Art:
Electrical home appliances such as television sets continue to undergo size and weight reduction. Coupled with this trend, significant size and weight reduction and performance improvements have also been made in connection with electric components such as motors and transformers. There is also a strong demand for improvements to their reliability.
From these standpoints, coating materials having excellent heat resistance are desired as coating materials for insulated wires employed in such electric components. Wires produced by using a polyester resin-base coating composition are used as type B (130.degree. C.) or type F (155.degree. C.) insulated wires. In addition, polyimide resins are also known to have excellent heat resistance.
Wires of smaller diameters are also needed for the size and weight reduction of electric components and for improvements to their performance. Polyester-insulated wires having such smaller diameters are now required to have still higher performance.
These polyester-insulated wires are used in severe environments, so that they are required to have high chemical resistance, solvent resistance, hydrolysis resistance and alkali resistance in addition to good heat resistance and electrical insulation.
Further, rationalization of their production process is also required to reduce the manufacturing cost.
Aspects of this rationalization include rationalization and line operation performance of stripping of insulation from the ends of insulated wires.
Stripping of insulation from the ends of insulated wires is currently done by various methods, including mechanical stripping, thermal decomposition stripping, chemical stripping and solder stripping. Solder stripping is however most preferred in view of the shorter working time, the protection of the conductors of small-diameter wires from damage, the possibility of continuous processing, etc.
Although the stripping of insulation from the ends of wires by a chemical reagent has already been carried out as a line operation, a relatively long time is required for the immersion in the chemical reagent and subsequent washing is indispensable. In addition, there is potential danger in that the chemical reagent must be handled. It has hence been attempted to conduct solder stripping as a line operation. However, none of conventional polyester-insulated wires were solder strippable.
As heat-resistant, insulated wires, there are polyimide-insulated wires [Publication of Unexamined Japanese Patent Application (KOKAI) No. 1-124905]. They however do not offer strippability.
As insulated wires which permit solder stripping, there are wires insulated by a polyurethane coating composed principally of a polyurethane resin as disclosed in Publication of Unexamined Japanese Patent Application (KOKAI) No. 64-90268. Their heat resistance is however as low as type E (120.degree. C.).
There is accordingly a strong demand for the development of insulated wires having a heat-resistant, resin coating whose heat resistance is at least type B (130.degree. C.).
When soldering an insulated wire strand composed of many wires, there is recently an increasing tendency to conduct its end processing in such a way that the insulated wire strand with its insulating coating still applied thereon is directly immersed in a solder bath to achieve simultaneous stripping of the insulating coating, and soldering.
For this purpose, the insulating coating must be removed as promptly as possible after the immersion in the solder bath. Needless to say, the shorter the immersion in the solder bath, the better.
With solder stripping, the temperatures of a molten solder bath higher than 450.degree. C. significantly accelerate oxidative deterioration of the solder bath and the velocity of dissolution of copper as a conductor into the solder. Such high temperatures therefore lead to the problem that the conductors of insulated wires may be reduced in diameter.
Proposed in view of such solder strippability as described above include insulated wires having an insulating coating of a polyester imide resin [Publication of Unexamined Japanese Patent Application (KOKAI) No. 63-289711], polyurethane compositions formed of a polyester imide and an isocyanate [Publication of Unexamined Japanese Patent Application (KOKAI) No. 63-69819], etc. They are however accompanied by the drawbacks that their prices are high because their chemical structures are different and/or they require special raw materials and/or a complex reaction and that they are inferior in electrical insulation because of a high concentration of polar groups. In addition, they also have the potential problem that they may have inferior solderability due to the inclusion of an ester-type crosslinking structure at a high concentration.